hvac controls 2003
TRANSCRIPT
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HVAC System controls
By
Samantha Gunawardana
HVAC- Heating, ventilation and air conditioning-
account for 70 to 80 % of a buildings operating
cost
High energy prices are driving building owners to
seek automation and cut costs.
Demands for better air quality are increasing
through legislation.
Use systems only on Demand Control systems
Energy Share
Mar 2010 Apr 2010
May 2010
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Why do we need a control system?
Basic controls: Form of Manual switching
Time switches
Thermostats
The purpose of a control system is to automate
and take control of these operations in most
efficient way for the occupiers
Fundamentals of Automatic controls
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Fundamentals of Automatic controls
Typically there are two types of control loopsused in HVAC applications.
Fundamentals of Automatic controls
Fundamentals of Automatic controls
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Fundamentals of Automatic controls
The controllers can be classified by the type of controlaction taken when the controlled variable deviates
from the set point.
Fundamentals of Automatic controls
Most common
Wide temp variation
Rapid cycling
More stable and less cycling
than two-position control
Fundamentals of Automatic controls
The amount of change in the
controlled variable, over the
full range of controlled
device is called the throttlingrange
Provides stable control
Inherent disadvantage is its
offset.
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Fundamentals of Automatic controls
Integral control overcomes
the offset characteristics of
the proportional control.
It responds not only to
magnitude of the deviation,
but also for how long it
exists.
Fundamentals of Automatic controls
Derivative control generates
a corrective output signal
only when the condition of
controlled variable is
changing.
Fundamentals of Automatic controls
PI controls is used in
applications where offset is
unacceptable. Good for
places where no rapidchanges in controlled
variable. Good for HVAC
applications
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Fundamentals of Automatic controls
Fundamentals of Automatic controls
Fundamentals of Automatic controls
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DDC Vs Conventional
Conventional controls are sometimes referred asSingle loop controls(SLC)
In Contrast Direct Digital Controls many control
sequences simultaneously.
DDC Vs Conventional
Control system Designer
Need to know about DDC
Need to know the at least basics of what equipment
to be controlled Related guidelines for those equipment
Must be a expert on every system that he or she
works with and seeks to control
Should be familiar with computer systems
Basics of electronics and electricity
Knowledge of control theory
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Input and Sensors
There are five specific type of sensors Temp Sensors(Bimetal, Rod and tube, RTD,
Thermistor, Thermocouple
Humidity sensors
Pressure sensors
Flow sensors
Special purpose sensor(watt, current, kw pulse,CO)
Input and Sensors
Sensor selection consideration
Environment
Operating range
Sensor signal compatibility
Set point accuracy and repeatability
Response time (Fast response is not always
desirable)
Controllers
Controller is a device that collects information from the sensor,
interprets this information, and send commands to controlled
device to effect a corrective action.
Types of controllers
Pneumatic
Electric
Electronic
Direct Digital
Hybrid
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Controlled Devices
Controlled devices are components that are controlled by
mechanical or electrical means
Common controlled devices are
Control Valves
Control Dampers
Control Valves(Two position, Proportional)
Mech engineer to design the constant pressure drop across the valve
There are three general classifications of flow characteristics
Equal percentage
Lineari ty
Quick Opening
Controlled Devices
Control Valves operator
To operate a motor control valve through its stroke a
controlled device called Actuator is needed.
Electrical solenoid (Plunger is connected to valve stem)
Electric Motor actuators (Utilise a gear train)
Birdirectional
Spring return
Reversible
Controlled Devices
Control Damper(A valve
for air)
Two Position Modulating control
Damper actuator
Rotation shaft designed to
link with damper
Electrical Auxiliary
devices
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Static pressure control
Purpose of static pressure control is to maintain minimum amount
of static pressure in duct system to support proper air distribution
Supply and return air flow control
Two primary modes of
controls are
Duct static pressure
Building static pressure
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Variable air volume control
The purpose of VAV terminal control is to modulate the cooling
supply air to meet the minimum cooling requirement of the space
Variable air volume control-contd
VAV box which is pressure dependant(no pressure control)
In environments where fluctuations in air volume is not critical
Variable air volume control-contd
VAV box which is pressure independant(with pressure control). Flow
controller controls the minimum flow requirement
In systems where a min qty of supply air must be provided under all conditions
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Supply air temperature reset control
The purpose of supply air reset temperature control is to reset the
supply air temperature when there is a reduced demand forcooling.
Chilled water temp reset control
Cooling Tower fan speed control
One of the most overlooked energy conservation strategies for
HVAC systems.
Control strategy: When demand
for condenser-water temperatureis reduced as a result of reduced
cooling load, modulate the speed
of the tower fan to maintain the
condensor water set point
Add Wet bulb temp
sensor/Humidity sensor
Lowering condenser water temp
allows chiller to operate more
efficiently
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Variable pumping strategies for
chilled water systemSingle Chiller with three way valves at load.
Most large chilled water system
have a diversity of 50% to 60%.
This contributes heavily to the
energy wasted in recirculating
unused water.
In fact, in commercial buildings
it is very common to find many
hours of low system loads while
mechanical plants continue to
serve the building at peak
capacity.
Variable pumping strategies for
chilled water systemSingle Chiller with three way valves at load and VFD.
Though VFD saves energy by
minimizing the unused
circulating water
However the less water to chiller
can create problems. Coefficient
of performance can be
drastically altered
Variable pumping strategies for
chilled water systemSingle Chiller with two way valves at load and By pass circuit
Constant chiller flow, variable system flow
Problems of low water flow to
chiller is solvedNO VFD, therefore no saving at
CHWP
Saving by chiller staging
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Variable pumping strategies for
chilled water systemMultiple Chiller configuration- Constant chiller flow, Varaible
system flow
Addition of a temp sensor
The relationship between return
temp and supply temp indicates a
surplus or deficit of water supply
compared with water demand.
Measuring the system load by
valve postions.
Good for building loads closer
to the central plant, so the
distribution pumping power is
not a matter of great concern.
Variable pumping strategies for
chilled water systemMultiple Chiller configuration-Primary and secondary chilled water supply loops
Chiller may operate independently under
constant flow
Distribution system will operate for the
building demand with variable flow
Flow switch to indicate the direction of flow
Flow meter measure the flow
Chiller plant operate under highly efficient
constant flow variable temp
Building load to operate in a varaible flow
constant temp which is most efficient inpumping capacity
Direct Digital control of air handling
systemVariable air volume system
Minimum outside air control, supply reset control, duct static pressure control,
and zone terminal control all act in concert to provide right quantity and
temeperature of air to the occupied space.
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Control of an HVAC system
Unit level control
Unit level control
Primary function is to protect
the equipment
Standalone-capable of
performing even if no support
from system level
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System Level control
Provides intelligent , cordinated
control
System Design Methodology
END